This invention relates generally to semiconductor device fabrication, and more particularly the invention relates to the formation of shallow junctions in a semiconductor device through the selective introduction of dopants.
Semiconductor devices and integrated circuits are fabricated through the selective introduction of dopant atoms into a semiconductor substrate. Conventionally, photoresist masking and chemical etching techniques are utilized to define diffusion windows for the dopant atom diffusion or ion implantation into the wafer.
With the reduction of dimensions of semiconductor devices and increased density of integrated circuits, shallower junctions are needed in the device structures. One method of obtaining shallow junctions is to outdiffuse dopants from a doped silicide layer into the underlying substrate using rapid thermal lamp annealing. The use of silicides minimizes contact and series resistances and prevents damage to the underlying silicon. Shorting problems which are encountered in the silicidation of preformed shallow junctions are prevented since the dopant profile in the silicon substrate follows the contour of the silicide/silicon interface.
Rapid thermal lamp annealing typically has a relatively long heat cycle (e.g., seconds) which limits the amount of dopant in shallow junction devices. Further, the entire wafer is heated, which can lead to warpage. Pulsed laser techniques have been introduced to overcome these limitations. In gas immersion laser doping (GILD) a suitably masked wafer of a semiconductor such as silicon is placed in a gas cell and immersed in a desired dopant gas such as BF.sub.3, ASF.sub.5, and PF.sub.5 / A pulsed, high-intensity laser is then used to heat the masked wafer with dopant atoms being driven into a shallow silicon surface region during a melt/regrowth of the exposed silicon surface.
The present invention modifies the gas immersion laser doping process to provide shallower junctions and better process control and less dopant per pulse along with less disruption of existing doped regions.